A planar light source device is incorporated in the liquid-crystal display devices used in, for example, personal computers, cell phones, and so forth, in order to respond to demands for greater thinness, lighter weight, greater labor savings, and higher definition. In addition, with the goal of functioning to uniformly and efficiently guide the incident light to the liquid-crystal display side, this planar light source device is provided with a flat plate-shaped light guide plate or a light guide plate that has a wedge-shaped cross-section in which one surface has a uniformly sloped surface. In some instances a concave-convex pattern is also formed in the surface of the light guide plate in order to provide a light scattering function.
Such light guide plates are obtained by the injection molding of a thermoplastic resin, and the aforementioned concave-convex pattern is generated by transfer from a concave-convex region formed in the surface of an insert. Light guide plates have in the past been molded from a resin material such as polymethyl methacrylate (PMMA); however, of late a conversion has been underway to the more highly heat-resistant polycarbonate resin materials due to the trends of demand for display devices that render sharper images and the higher temperatures within the device caused by the heat generated in proximity to the light source.
Polycarbonate resins exhibit excellent mechanical properties, thermal properties, and electrical properties and an excellent weatherability; however, they have a lower light transmittance than PMMA and the problem thus arises of a lower brightness when a planar light source assembly is constructed of a light source and a polycarbonate resin light guide plate. In addition, there has lately been demand for a small chromaticity difference between the incident light area of the light guide plate and locations distant from the incident light area, but a problem here is that polycarbonate resin more readily undergoes yellowing than does PMMA resin.
A method is proposed in Patent Document 1 in which the light transmittance and brightness are improved by the addition of an acrylic resin and an alicyclic epoxy; a method is proposed in Patent Document 2 in which the brightness is improved by modifying the polycarbonate resin terminals and raising the transferability of the concave-convex region to the light guide plate; and a method is proposed in Patent Document 3 in which the brightness is improved by improving this transferability by introducing a copolyester carbonate that has an aliphatic segment.
However, in the case of the method in Patent Document 1, while the addition of the acrylic resin does bring about a good hue, the light transmittance and brightness cannot be raised due to the appearance of cloudiness. The addition of alicyclic epoxy may improve the transmittance, but a hue-improving effect is not recognized for this. In the case of the methods in Patent Document 2 and Patent Document 3, an improvement in the flowability and transferability can be expected, but the problem occurs of a reduction in the heat resistance.
On the other hand, the incorporation of, for example, a polyethylene ether glycol or a poly(2-methyl)ethylene ether glycol in thermoplastic resins such as polycarbonate resins is known, and Patent Document 4 describes a 7-radiation resistant polycarbonate resin that contains same, while Patent Document 5 describes a thermoplastic resin composition that is provided by incorporating same in, for example, PMMA, and that has an excellent static inhibition and an excellent surface appearance.
Patent Document 6 proposes that the transmittance and hue be improved through the incorporation of a polyethylene ether glycol or poly(2-alkyl)ethylene ether glycol with the formula X—O—[CH(—R)—CH2—O]n—Y (R is the hydrogen atom or a C1-3 alkyl group). Some improvement in the transmittance and yellowing (yellow index: YI) is seen due to the incorporation of the polyethylene ether glycol or poly(2-alkyl)ethylene ether glycol.
However, the trends toward further thinning and thinning in large sizes have in particular been developing quite rapidly recently with regard to various mobile terminals such as smartphones and tablet type terminals, and light injection into the light guide plate has been carried out from the lateral edge of the light guide plate rather than from the back of the light guide plate and a satisfactory brightness has come to be required from ultrathin light sources. In such high-end light guide plates, the transmittance and YI level achieved by the above-described prior art do not satisfy the required specifications.
In addition, because, in the case of polycarbonate resins for light guide applications, thin-wall molding is carried out at higher temperatures than ordinary molding temperature of polycarbonate resins, higher fluidities by reducing the viscosity-average molecular weight are requested even at the sacrifice of the mechanical strength. Polycarbonate resins for thin optical components, as represented by these light guides, are thus materials with a weaker mechanical strength than conventional polycarbonate resins, and as a result during pellet production with an extruder, the extruded polycarbonate resin strand ends up being prone to breakage during cooling and the problem arises that stable production is impaired.
Moreover, the produced pellets are introduced at the plant into, e.g., paper bags or flexible containers for delivery, and even during transport alone a portion is converted into fines due to pellet-to-pellet contact. When pellets containing such fines are used for the molding of, for example, a light guide, the problem then arises that yellowing and optical fluctuations are readily occurred in the molded article.
With regard to solving the problems caused by fines, it can be solved by removing the fines by passage through a fines-removal device during the molding process. However, this creates a contamination risk due to the introduction of an extra step, and thus there is a desire to avoid this as much as possible.
In order to prevent the generation of silver streaks in optical discs, Patent Document 7 provides a polycarbonate resin pellet for optical disc applications, for which the average value of the pellet length is in the range from 2.5 to 3.5 mm and at least 70% thereof is in the range of the average value of the length±0.1 mm. This document states that such pellet with little fine powder avoids incorporating air during plasticization and that an optical disc substrate free of silver streak is obtained; however, no description is provided with regard to the shape of these pellets.
In order to achieve a shortening of the molding cycle for optical disc substrates, Patent Document 8 provides a polycarbonate molding material for optical disc substrates, characterized in that the average value of the pellet length is in the range from 2.5 to 3.5 mm, the average value of the major diameter of the ellipse cross-section is 2.60 to 3.2 mm, and at least 70% of the pellets reside in the range of the average value of the length±0.08 mm and in the range of the average value of the major diameter±0.12 mm. This document states that, by having the length and major diameter of the pellets be in the indicated ranges, a three-dimensional shape is obtained in which the ratio between the length and major diameter takes on a balanced condition of approximately 0.7 to 1.5 and its distribution resides within a specified and narrow range and because of this a very uniform shape is assumed. It is further stated that as a result the shape is better adapted to the structure of the cylinder and screw of the injection molding machine used for discs and the melting efficiency during plasticization is raised, the plasticization time is then shortened, and the production of optical disc substrates by what is known as high-cycle molding, which has a short molding cycle, is made possible. While another characteristic feature in Patent Document 8 is that the pellet mass has a uniform shape, there is no description of the details of the elliptical shape of the individual pellets, and in addition only production by simply cutting the strand is described with regard to the specific method of producing these pellets.
In addition, as in the inventions described in these Patent Documents 7 and 8, minimizing the fines in the pellet mass is also critical for thin optical components such as light guides, but this by itself is not sufficient for polycarbonate resin pellets for thin optical components in which the occurrence of yellowing and optical fluctuations is suppressed.